1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for performing a marine seismic survey using buoys that carry appropriate seismic sensors.
2. Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of geophysical structures under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration for determining properties of earth's subsurface, which is especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional system for generating the seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed horizontally, i.e., lying at a constant depth relative to the surface 14 of the ocean. The streamers may be disposed to have other than horizontal spatial arrangements. The vessel 10 also tows a seismic source array 16 configured to generate a seismic wave 18. The seismic wave 18 propagates downward toward the seafloor 20 and penetrates the seafloor until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver 11 on the streamer 12. Based on the data collected by the receiver 11, an image of the subsurface is generated by further analyses of the collected data.
However, this traditional configuration is expensive because the costs associated with operating the towing vessel and the streamers are high. In addition, the data produced by the receivers of the streamers is poor due to the flow noise produced by the movement of the streamers in water. Further, the notch diversity of the data recorded with the streamers might be limited. To overcome some of these problems, new technologies deploy seismic sensors on the bottom of the ocean (ocean bottom stations, OBS) to improve the coupling and reduce the noise. Even so, positioning the seismic sensors remains a challenge for OBS technology.
Other technologies use permanent receivers set on the sea bottom, as disclosed in U.S. Pat. No. 6,932,185 (herein '185), the entire content of which is incorporated herein by reference. In this case, the seismic sensors 60 are attached, as shown in FIG. 2 (which corresponds to FIG. 4 of the '185), to a heavy pedestal 62. A station 64 that includes the sensors 60 is launched from a vessel and arrives, due to its gravity, at a desired position. The station 64 remains on the bottom of the ocean permanently. Data recorded by sensors 60 is transferred through a cable 66 to a mobile station 68. When necessary, the mobile station 68may be brought to the surface to retrieve the data.
Although this method provides a better coupling between the ocean bottom and the seismic receivers, the process is still expensive and not flexible because the stations and corresponding sensors are difficult to move around or reuse. Further, positioning the stations is not straightforward. Furthermore, the notch diversity is not greatly improved.
An improvement to this method is described, for example, in European Patent No. EP 1 217 390 (herein '390), the entire content of which is incorporated herein by reference. In this document, a receiver 70 is removably attached to a pedestal 72 together with a memory device 74 as illustrated in FIG. 3. After recording the seismic signals, the receiver 70 and the memory device 74 are instructed by a vessel 76 to detach from the pedestal 72 and to surface at the ocean surface 78 to be picked up by the vessel 76.
However, this configuration is not very reliable because the mechanism maintaining the receiver 70 connected to the pedestal 72 may fail to release the receiver 70. Also, the receiver 70 and pedestal 72 may not reach their intended positions on the seabed. Further, the fact that the pedestals 72 are left behind increases ocean pollution and the survey price, which is undesirable.
Thus, it can be seen from above that a characteristic of existing methods is the record of seismic signals either (i) close to the surface, with streamers, or (ii) at the seabed with OBS. Neither situation offers the desired notch diversity.
Accordingly, it would be desirable to provide systems and methods that provide an inexpensive and reliable device for recording seismic signals with good notch diversity.